[link to www.dailymail.co.uk] Brazils biggest drug cartel has promised a 'World Cup of terror' next year - in a reminder of the high level of violence that still marks the country.The threat was issued by the First Capital Command in Sao Paulo, who last year was behind the murder of more than a hundred of the city's police officers.And last night violent demonstrations exploded on the streets of Rio de Janeiro and Sao Paulo with protesters setting fire to cars and fighting pitched battles with police.The protests were in support of a teachers' strike and were the latest in a long line of unrest that has seen millions take to the streets in Brazil to protest against the government.Both will be a sobering reminder amid the euphoria of England's fans that next year they will be travelling to a country where safety and stability cannot be guaranteed.Six matches in next year's competition, including the opening game, will be held at the Arena de Sao Paulo while seven, including the final, will be held in Rio de Janeiro's Estadio do Maracana.In messages intercepted by police this week, leaders of the gang in Sao Paulo made the vague but ominous threat should the authorities move jailed members of the cartel to a tougher prison.In Brazil, powerful gangs often linked to the drug trade are very powerful and frequently control whole prisons and favelas, or shanty towns.Scroll down for video